1. Field of the Invention
The invention relates to a preparation method of a quantum dot, and more particularly to a preparation method of a quantum dot capable of emitting white light.
2. Description of the Related Art
Currently, preparation method of phosphor includes sintering, sol-gel and micro-wave hyperthermia etc. For sintering, diffusional reaction between oxides is accelerated by a hyperthermal temperature (>1000° C.) to prepare phosphor. Diffusional reaction between oxides, however, is slow and requires a long time. For sol-gel, a precursor is formed at a low temperature and a solid state diffusional reaction is then executed by heating the precursor up to a hyperthermal temperature to produce phosphor. Thus, requiring an additional step along with the hyperthermal temperature for preparation. For micro-wave hyperthermia, the solid diffusional reaction is executed by micro-wave with a character of rapid local heat, as a heat source. By the above methods, however, only phosphor with monochromatic light is prepared.
For traditional phosphor, since lattice field effect is different from expansion of electron cloud, adding various active elements to modify its photoluminescence wavelength is required. Moreover, because luminous mechanism of phosphor belongs to energy transfer between molecular orbitals (the unlike orbitals like f-d or the like orbitals like f-f), phosphor is excited only by light with a certain wavelength and energy loss is large.
Typically, phosphor is mainly utilized in colorization, namely luminous. Phosphor is widely applied in light-emitting diodes (LED) with character of energy conservation, for instance. As petroleum and other energy sources from natural resources dissipate, popularity of energy conservation by using LED products is gaining momentum, especially white light LEDs (WLED).
At present, fabrication of WLED mainly includes two methods. The first method utilizes a plurality of LED chips, namely, red, green and blue LED chips are packaged together, and white light can be obtained by mixing the individual lights. The second method, generally, is called a single LED chip. Various prearranged phosphors are excited by a single LED chip, and the white light can be obtained by mixing the light of chip and phosphors. For example, yellow phosphor such as yttrium aluminum garnet (YAG) can be excited by a single blue LED chip to produce yellow light, then, the yellow light can be mixed with blue light emitted by the blue LED chip to produce white light. Similarly, the prearranged blue, green and red phosphors can be excited by an LED chip capable of emitting ultraviolet light to mix with and produce white light.
Meanwhile, for an ultraviolet LED chip type, in order to achieve white light, mixing of red, green and blue phosphors is required and fabrication and design of the WLED is complicated because luminous efficiency and decay rate of phosphors are different.
Since conventional phosphors are monochromatic light, in order to obtain white light, mixing two or more phosphors is required so that costs and fabrication difficulty of WLEDs are increased.
Thus, a material capable of emitting white light is desired to reduce fabrication costs and eliminate the previously described shortcoming.